Pressurized-water nuclear reactors generally include a vessel enclosing the reactor core which is submerged in the pressurized water for cooling the core.
The reactor vessel, of cylindrical overall shape, includes a head of hemispherical shape which may be attached to its upper portion. The head is pierced with openings at each of which is fixed, by welding, a tubular penetration part constituting an adaptor providing for the passage of the drive for moving a follower of a rod for controlling the reactivity of the core or a passage for penetration of a measuring means into the core, such as a column of thermocouples.
The mechanisms for actuating the movement of the rods for controlling the reactivity of the core are fixed to the end portions of each of the adaptors.
A thermal sleeve is fixed inside each of the tubular penetrations of the vessel head, in a position coaxial in favor of the tubular penetration part and with a certain radial clearance, this thermal sleeve including a diametrally enlarged portion resting on a diametrally enlarged hearing surface located at the upper portion of the bore of the tubular penetration part and which is mounted so as to rotate freely inside the penetration.
The followers of the rods for controlling the reactivity of the nuclear reactor penetrate the vessel head, inside the thermal sleeves which are themselves arranged coaxially inside the adaptors of the control rods or, more generally, inside the tubular penetration parts of the head.
In order to increase the reliability and the operating safety of the nuclear reactors and in order to extend the lifetime of these reactors, the plant operators are required to carry out increasingly numerous inspections of the various elements constituting the nuclear reactor and repairs of these elements when defects are detected.
In particular, it may be necessary to inspect the state of the penetration parts of the head of the vessel in order to assure the integrity of these parts, after the reactor has been operated for a certain time, in particular in the zone where these tubular parts are welded to the vessel head.
Various inspection methods and devices have been proposed enabling defects on the inner surface of the tubular penetration part to be detected and the defects which have been detected and located to be repaired.
In particular, methods and devices have been proposed making it possible to carry out the inspection by using ultrasound or eddy currents or even by using a liquid-penetration method with remote endoscopic examination of the internal surface of the penetration part, such as an adaptor.
In order to carry out the repair of the zones exhibiting defects and, in particular, cracks extending over a certain depth into the wall of the adaptor, from its inner surface, various methods have been proposed, such as cutting away by machining followed by a build-up in the defective zone, cutting away the defects by machining without build-up, or coating with a layer such as an electrolytic nickel layer.
Inspections or repairs carried out on the internal surface of the adaptor most often require dismantling of the thermal sleeve in order to provide access to the internal surface of the bore of the adaptor.
Inspections and repairs are carried out during a shutdown of the nuclear reactor, the head of the vessel being dismantled and placed on a maintenance/repair stand.
Most repair methods used to date require the identification and precise location of the cracks, as well as a determination of the depth of penetration of the cracks in the wall of the adaptor. It may be necessary to carry out several machining passes with an inspection of the machined zone between two successive machining passes.
Furthermore, when a repair by build up turns out to be necessary, after cutting away the surface of the adaptor, stresses are created in the metal deposited by build-up and in the metal of the adaptor in the vicinity of the built-up zone. These stresses may cause new cracking of the adaptor, after the head of the vessel has been put back into service and after the reactor has been operated for a certain period.
More generally, it may be necessary to use an effective method for repairing defective zones, elements of a nuclear reactor other than adaptors and, in particular, elements of tubular shape such as the sleeves for penetrating the vessel bottom head of a pressurized-water nuclear reactor providing for the passage of instrumentation guide conduits. Such a repair method has to be able to be implemented remotely, very safely and with as few inspection operations as possible.
The repairs carried out must also make it possible to prevent the appearance of new cracking in the repaired zones or in their vicinity.